M. Heydari-Malayeri - Paris Observatory

An imaginary sphere, of large but indefinite dimension, used as a
basis to define the position coordinates of celestial bodies. The center can
be the Earth, the observer, or any other point which plays the role of origin
for a given system of coordinates. Seen from the Earth, the celestial sphere rotates
around the → celestial axis every 23h 56m 04s
(the → sidereal day), as a
result of the Earth's rotation. Two important circles on the celestial
sphere are the → celestial equator and the
→ ecliptic. The angle between them, about 23.40 degrees, is known
as the → obliquity of the ecliptic. The celestial equator
and the ecliptic intersect at two points, → vernal equinox
and → autumnal equinox. The positions of the
→ celestial poles and therefore that of the
→ celestial equator move gradually on the celestial
sphere, due to → precession.